1. Technical Field
The present invention relates to a wear-resistant layer, which is preferably used on piston rings for internal combustion engines. In particular, a wear-resistant layer in the form of a coating for such a piston ring is described as well as a method for applying the wear-resistant layer and for producing such a piston ring with a wear-resistant layer of such kind.
2. Related Art
Piston rings are sealing elements on the piston of an internal combustion engine or a piston compressor, for example. In an internal combustion engine, piston rings seal the gap between the piston head and the cylinder wall with respect to the combustion chamber. As the piston moves up and down, the outer peripheral surface of the piston ring slides along the cylinder wall, in constant springy contact therewith, and at the same time, because of the tilting movements of the piston, the piston ring oscillates in its piston ring groove, so that the flanks of the ring are alternately in contact with the upper and lower flank of the piston ring groove. In each case where friction bodies are sliding against one another, a certain amount of wear inevitably occurs, which may be more or less heavy depending on the material, and if the parts are unlubricated, this may result in scuffing scoring, and ultimately the destruction of the engine.
For manufacturing combustion engine parts that are subject to heavy stresses, such as piston rings, the most commonly used materials are cast iron and cast iron alloys. Piston rings, particularly compression rings are exposed to ever increasing loads in highly stressed internal combustion engines such as 4-stroke and 2-stroke engines. Such loads include a high peak compression pressure, a high combustion temperature and a reduction of lubricant film on the piston ring, all of which have a significant impact on functional properties such as wear, scuffing resistance, microwelding and corrosion resistance.
High ignition pressures, lower emissions and direct fuel injection represent further intensification of the loads on piston rings. The consequences are damage and plating of piston material, particularly on the lower piston ring flank.
As the mechanical and dynamic loads placed on piston rings continue to increase, more and more engine manufacturers are demanding piston rings made from high quality steel (tempered and high-alloyed like the material 1.4112). In this context, iron materials that contain less than 2.08% by weight carbon are referred to as steel. If the carbon content is higher, the material is called cast iron. Steel materials have better strength and toughness properties than cast iron, because there is no interference by free graphite in the basic microstructure. The most commonly used materials for manufacturing piston rings are martensitic steels with high chromium alloyed content.
In order to further improve the sliding and wear characteristics of piston rings with respect to the cylinder wall, the peripheral surface thereof is provided with coatings of various materials. Such coatings contain, for example, molybdenum, chromium, nickel, boron, silicon, aluminium, copper and/or carbon. One commercially available piston ring coating is known by the name MKP200 and contains a composite of molybdenum and Cr2C3—NiCr.
However, piston rings with such coatings are not ideally equipped to meet the demands of future engine generations in terms of resistance to wear, scuffing and corrosion. One reason for this is the high porosity of the protective layer, which is in the order of about 10-15%, and in a limitation on the proportion of wear-resistant components in the layer due to the spraying method used.